The eutectic mixture of diphenyl oxide and biphenyl has been widely used as a high temperature heat transfer fluid for more than forty years (Grebe, U.S. Pat. No. 1,882,809). Among its more critical properties are its high boiling point, its thermal stability at and above the boiling point in both the liquid and the vapor phase, and the fact that the liquid and vapor phases have substantially the same composition. A primary drawback of this binary eutectic mixture is its relatively high freezing point (54.degree.F.). This complicates its use in many outdoor applications.
In order to improve its utility, many additives have been proposed for the mixture. Among these are naphthalene, aniline, diphenylmethane and chlorodiphenyl (Rittler et al., U.S. Pat. No. 2,149,789). These have not been satisfactory, for one reason or another. Some raise the freezing point. Others are unstable at high temperatures or in the vapor phase. Still others have boiling points so different from that of the binary eutectic that the vapor composition differs widely from that of the liquid.
The alkylated diphenyls have been known for almost as long as the above binary eutectic and have been suggested for use as heat transfer fluids (Krase, U.S. Pat. No. 2,172,391) but have not been combined with the eutectic to improve their respective properties.